production of prompt photons associated with jets at lhc...

Production of prompt photons associated with jets at LHC in kT-factorization

Maxim Malyshev1

in collaboration withHannes Jung2

Artem Lipatov1

1SINP, M.V. Lomonosov Moscow State University2DESY, Hamburg

Phys.Rev. D100 (2019) 034028

2

Maxim Malyshev QFTHEP`2019, September, 27, 2019

Outline

1. Motivation2. kT-factorization approach3. Numerical results4. Conclusion

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Motivation

Prompt photons — produced in the hard subprocess (rather than in meson decays);

Insensitive to the final-state hadronisation — direct probe of the hard subprocess dynamics;

test of different parton distribution functions in proton;

Background to many SM and BSM processes;

Photon+jet production allows to study correlation observables, and through them — production mechanisms and evolution details.


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Motivation

kT-factorization approach — efficient tool to study high energy physics processes, based on BFKL or CCFM evolution equations;

CASCADE3: kT-factorization with parton showers.

In this work we include for the first time parton showers for prompt photon production in kT-factorization.


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kT-factorization

Main ingredients:

Off-shell matrix elements

TMD (unintegrated) parton densities.

The cross-section:


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kT-factorization: off-shell matrix elements

Off-shell gluon polarization sum (BFKL rule):

Reggeized partons

q*qγ-vertex:


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kT-factorization: off-shell matrix elements

BCFW recursion + method of auxilliary quarks (KaTie [A. Van Hameren, Comput.Phys.Commun. 224 (2018) 371]).


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kT-factorization: TMDs

1) KMR prescription at LO and NLO (MRW)A procedure to introduce kT at the last step of DGLAP

evolution [M. Kimber et al., Phys.Rev. D94 (2001) 114027, Eur.Phys.J. C31 (2003) 73; A.D. Martin, et al., Eur.Phys.J. C66 (2010) 73].


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kT-factorization: TMDs2) CCFM-based unintegrated distributionsNumerical solutions of Catani-Ciafaloni-Fiorani-

Marchesini evolution equation.

The starting distribution is chosen to satisfy data on proton structure functions F2(x,μ

2) only (A0, JH2013-set-1) or both F2(x,μ

2) and F2c(x,μ2) (JH2013-set-2)

[H. Jung, hep-ph/0411287, F. Hautmann, H. Jung, Nucl. Phys. B883

(2014) 1].

Only gluons and valence quarks. Sea quarks can be obtained from gluons in the last splitting.


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Isolation criterion

Standard isolation experimental cuts:

ET

hadEmax

(ηhad − η)2 + (φhad − φ)2 ≤ R2

significantly reduces fragmentation contributions (so they are not taken into account in the work)


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Prompt photons studies with kT-factorization

● Inclusive production

A.V. Lipatov, N.P. Zotov, J.Phys. G34 (2007) 219;S.P. Baranov et al. Phys.Rev. D77 (2008) 074024 (first calculation for g* + g* +Q + Q);A.V. Lipatov, M.A.M., Phys.Rev. D94 (2016) 034020.

● Prompt photons with heavy quark jets production

Heavy quark jets originate from hard subprocess, rather than from the initial state radiation (ISR)

A.V. Lipatov et al. JHEP 1205 (2012) 104 (better description of D0 and CDF data, than within collinear approach);V.A. Bednyakov et al. Eur.Phys.J. C79 (2019) 92.

Good description of HERA, Tevatron and LHC data


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Prompt photons studies with kT-factorization

● Prompt photons with jets photoproduction

Hadron jets can originate from both ISR and hard subprocess

A.V. Lipatov, N.P. Zotov, Phys.Rev. D81 (2010) 094027;A.V. Lipatov et al. Phys.Rev. D88 (2013) 074001 (ISR jets are taken into account with the method of [S.P. Baranov, N.P. Zotov, Phys. Lett. B491 (2000) 111]);B.A. Kniel et al. Phys.Rev. D89 (2014) 114016.

Reasonable description of HERA data was achieved


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Prompt photon with jets, hadroproduction

kT-factorization for γ+ jet in hadroproduction:

T. Pietrycki and A. Szczurek, Phys.Rev. D76 (2007) 034003;

ISR jets were absent

A.V. Lipatov and N.P. Zotov, Phys.Rev. D90 (2014) 094005;

First attempt to take into account ISR jets in kT-factorization

for γ+ jet hadroproduction with the method of [S.P. Baranov, N.P. Zotov, Phys. Lett. B491 (2000) 111]


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«Naive» approach to obtain ISR jets [S.P. Baranov, N.P. Zotov, Phys. Lett. B491 (2000) 111]:


jet`s rapidity is unknown: taken randomly in some interval


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Theoretical setup

Subprocess taken in kT-factorization:

g* + g* +q + q

Calculated with newly developed Monte-Carlo generator PEGASUS (A.V. Lipatov, S.P. Baranov, M.A. Malyshev, in preparation, available soon)

Subprocesses taken in collinear factorization:

qv + g +q

q + q +g

q + q + q' + q'

q + q' +q + q'

● JH2013 set 1 and 2


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● Theoretical uncertainties are connected with the choice of the factorization and renormalization scales. We took μR = ξETγ. For JH2013 TMDs we took μF2 = (s+QT2), where s and QT2 are the energy of scattering subprocess and transverse momentum of the incoming off-shell gluon pair, respectively. We varied the scale parameter ξ between 1/2 and 2 about the default value ξ = 1.

● We use 2-loop (in kT-factorization) or 1-loop (in collinear case) formula for the strong coupling constant αs(μ2) with nf = 4 active quark flavors at QCD = 200 MeV. αem=1/137.

● Parton showers are produced with CASCADE (in kT-factorization) or Pythia (in collinear case).

● We use anti-kT-algorythm to construct jets with FastJet.

Parameters


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Numerical results: γ+ jet

√S=7 TeV

Dashed: JH2013set1 (ISR)

Dash-dotted: «naive approach»


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√S=7 TeV

Dashed: JH2013 (ISR)



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√S=7 TeV

Dashed: JH2013 (ISR)



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Conclusion

- We have significantly improved the scheme to study jet associated prompt photon production with taking into account subleading quark-subprocesses and TMD parton showers.

- Good description of ATLAS and CMS data has been achieved.

- The processes are sensitive to the TMDs.

- The calculations will be implemented in the new code PEGASUS.


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Back up


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From CMS Coll., JHEP 1406 (2014) 009.


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√S=8 TeVSolid: CCFM A0 predictions

Dash-dotted: KMR predictions

From A.V. Lipatov and N.P. Zotov, 2014


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kT-factorization: TMDs

3) PB unintegrated distributions

Unintegrated distributions are produced in the Parton branching method of the solution of DGLAP equation, by keeping kinematics during the solution process with angular ordering condition. [F. Hautmann et al. Phys.Lett. B772 (2017) 446; JHEP 1801 (2018) 070; Phys.Rev. D99 (2019) 074008]


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Divergencies

● We do not use the concept of fragmentation functions obviously. In our approach the effect of final state radiation is already included in calculations at the level of partonic subprocess matrix elements (we have a 2 → 3 rather than 2 → 2 subprocesses). But as in the traditional approach the calculated cross sections can be split into two pieces: the direct and fragmentation contributions. They depend from fragmentation scale.

● In our calculations we take a scale μ as the invariant mass of the produced photon and any final quark and we restrict direct contribution to μ ≥ M = 1GeV in order to eliminate the collinear divergences in the direct cross section. Then the mass of light quark mq can be safely taken as zero. The numerical effects of M is really small. It is less important than other theoretical uncertainties (connected with choice of renormalization and factorization scales).


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Fragmentation contributions


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Definitions


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PEGASUS

● parton level Monte-Carlo event generator for pp processes;

● can work with TMDs;

● a lot of implemented processes (heavy quarks, quarkonia, etc.);

● can generate an event record according to the Les Houches Event (*.lhe) format;

● an easy way to implement various kinematical restrictions;

● compatible with HEPData repository https://www.hepdata.net;

● built-in plotting tool PEGASUS Plotter


https://www.hepdata.net/

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production of prompt photons associated with jets at lhc...

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